According to Dalton’s theory, matter is made up of an atom, and atoms are indivisible. Later, many scientists came forward proving that an atom is divisible and it consists of charged subatomic particles. Accordingly, numerous models of atoms were suggested. The modern model of the atom is based on Rutherford-Bohr model. In 1913, Neils Bohr suggested a planetary model of the atom. He suggested that electrons are distributed in concentric circular orbits while nucleus is the center. Let us learn more about the electronic configuration of an atom and how to write the electronic configuration of elements.
According to Neils Bohr’s postulates, electrons are revolving around the nucleus along a specific pathway called orbits/shells. He called these orbits/shells as energy levels and represented them by letters and numbers like K, L, M, N, O,… and n= 1, 2, 3, 4,…respectively.
Neils Bohr and Bury suggested the distribution and arrangement of electrons in different shells. The distribution of electrons among the different orbits/shells and subshells is known as the electronic configuration of an atom. Generally, the electronic configuration represents the electron distribution, when an atom is at the ground state. There are certain rules to be followed for the distribution of electrons among different shells.
Rule 1: The maximum number of electrons that can be accommodated in a shell is represented by the formula 2n2, where ‘n’ is the shell number. The distribution of electrons in different orbits is as follows:
K-shell/ 1st orbit (n=1) = 2(1)2 = 2
L-shell/ 2nd orbit (n=2) = 2(2)2 = 8
M-shell/ 3rd orbit (n=3) = 2(3)2 =18; and so on.
Rule 2: The outermost orbit can accommodate a maximum of 8 electrons.
Rule 3:The inner shells should be filled before accommodating the electrons in the outer shells. Electrons are filled in ascending order i.e., K-shell should be filled with the maximum number of electrons it can accommodate before the filling of L shell.
Let’s illustrate the electronic configuration of hydrogen, oxygen, and chlorine atoms.
Hydrogen Electronic configuration:
Atomic number= 1
K-shell/ 1st orbit: n=1
Oxygen Electronic configuration:
Atomic number= 8
K-shell/ 1st orbit = 2
L-shell/ 2nd orbit = 6
Chlorine Electronic configuration:
Atomic number= 17
K-shell/ 1st orbit = 2
L-shell/ 2nd orbit = 8
M-shell/ 3rd orbit = 7
According to the way in which how electrons are distributed among orbitals the stability of the orbitals also varies.
Also, we know that elements are arranged in the periodic table based on their electronic structure. With the arrangement on the basis of Electronic Configuration in Periods and Groups, elements with the same behavior can be identified easily.
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